This invention relates to a method and apparatus for controlling the voltage and load distribution as well as for synchronizing several inverters, the outputs of which can be connected to a common bus bar.
Such a method is known, for instance, from the paper "Interruption-free Power Supply with Inverters", Siemens-Zeitschrift 47 (1973); no. 2, pages 123 to 126. In this reference, problems arising in connecting inverters in parallel are discussed in conjunction with interruption-free power supplies, but similar problems also arise in other applications of parallel connected inverters, for instance, in emergency power supplies or converters.
The public power supply system is not always suitable for supplying sensitive consumers, since its availability within narrow tolerances, particularly with respect to voltage amplitude, is not always assured. Thus, the operation of data processing installations, for instance, can be disturbed by brief interruptions of a few 100 msec duration. To circumvent these difficulties, sensitive consumers are supplied in an interruption free manner via static converters in conjunction with an energy source which is independent of the supply network. This source is generally a battery which is charged by a controlled rectifier from the public power supply system. An inverter then supplies the sensitive a-c loads. To increase reliability, several inverters are often connected in parallel via a bus bar, resulting in a redundant design. Then, provisions must be made to keep the frequency and the voltage constant; the inverters must be synchronized with each other and the load must be distributed as uniformly as possible over all parallel operated systems. These conditions must be met for a number of operating conditions, i.e., not only in normal operation with all inverters in parallel, but also, for instance, if an individual inverter fails, and for operation with network reconnection.
The problems described arise in a similar manner also with parallel connected static converters and with emergency power supplies comprising several parallel connected inverters.
From the above-mentioned paper "Interruption-free Power Supply with Inverters", Siemens-Zeitschrift 47 (1973), no. 2, pages 123 to 126, a control device for parallel connected inverters is known, in which each inverter has a synchronizing device which determines the phase of the inverter assigned to it and compares it with the phase of the other inverters. To distribute the load in a defined manner, a control is provided which carries out a corresponding load transfer from the operating data of the parallel operating inverters via the magnitude setting input for the voltage of the inverter. In addition, a voltage control for the inverter must be provided. To adapt the inverter to different operating conditions, extensive intervention into the control apparatus must be made, which can lead to control oscillations when switching operating conditions.
From the publication 536, "Redundant Synchronization of Static Power Supply Installations" of July, 1975, of AG fuer industrielle Elektronik, Losone-Locarno, pages 3 to 7, bus bars of parallel connected inverters can be used as a synchronization line. The use of the bus bar for synchronizing, however, is called disadvantageous and is not discussed further.